1. Field of the Invention
The present invention relates to a device using a metal-insulator transition (MIT) effect, and more particularly, to a device using an abrupt MIT material as a conductor whose phase can be transformed into a metal.
2. Description of the Related Art
As is generally known, an MIT occurs at a Mott insulator and a Hubbard insulator. The Hubbard insulator is a consecutive MIT. A field effect transistor (FET) that uses the Hubbard insulator as a channel layer was introduced in an article by D. M. Newns et al., Appl. Phys. Lett., vol. 73, p. 780, 1998. Since the Hubbard insulator uses an MIT that occurs consecutively, charges to be used as a carrier need to be added consecutively until an excellent metallic characteristic is achieved.
An article by Hyun-Tak Kim, NATO Science Series Vol. II/67, Kluwer, p. 137, 2002, which is also described on the web site http://xxx.lanl.gow/abs/cond-mat/0110112, teaches a theory that supports an abrupt MIT due to the Mott insulator. According to the theory in the above article, the Mott insulator has a bounded and metallic electron structure. Energy between electrons of the Mott insulator is made to change, and thus, an insulator-to-metal transition does not occur consecutively; rather it occurs abruptly. Changing temperature, pressure or an electric field instigates the energy change between the electrons of the Mott insulator. For instance, when holes with a low doping density are added to the Mott insulator, the insulator-to-metal transition occurs abruptly or suddenly.
In a typical abrupt MIT device, when an inconsecutive MIT occurs, a large amount of current flows abruptly. Thus, an MIT material layer is more likely to be deteriorated. FIG. 1 illustrates a top view of a typical MIT device 10.
The typical MIT device 10 includes a pair of electrodes 14 and 16 arranged to be spaced a predetermined distance apart from each other on certain regions of a substrate 12. An MIT material layer is disposed between the pair of the electrodes 14 and 16. The MIT material layer makes an electric connection between the electrodes 14 and 16 and causes an abrupt MIT. The abrupt MIT causes the MIT material layer to be transformed into a metal layer. Hence, the MIT material layer can be used as an MIT material layer 18. The MIT material layer 18 has a width of ‘W’.
FIG. 2 illustrates a top view of a sample 20 analyzed by micro-Raman spectroscopy to check structural uniformity of the MIT material layer 18 of the typical MIT device 10 (FIG. 1). FIG. 3 illustrates a graph of the intensity as a function of the Raman shift for the MIT material layer 18 illustrated in FIG. 2. As is well known, Raman spectroscopy is used to observe vibration energy of lattices. For metal, a peak is not observed. The width W of the MIT material layer 18 is exaggerated for clarity.
The sample 20 includes the MIT material layer 18 disposed on a support 22 and an analytical electrode 24 segmented into a certain number of regions and contacting the MIT material layer 18. For instance, the analytical electrode 24 is segmented into three regions including an upper region 24A, a central region 24B and a lower region 24C and has a protruding structure. In FIG. 3, the intensity as a function of the Raman shift, which is typically reported in units of cm−1, shows the characteristics of a substrate, more particularly, a region a representing Al2O3, and regions b and c measured when a large amount of current, e.g., a current amount of ‘F’ as marked in FIG. 8, flows through the MIT material layer 18. The curvature regions b and c are the measurements at the central region 24B (see FIG. 2) and at the upper and lower regions 24A and 24C (see FIG. 2), respectively. Reference numeral 35 represents a peak indicating Al2O3. Scattered Raman peaks indicate that a phase of the MIT material layer 18 is not yet transformed into a metal state. Therefore, the MIT does not yet take place at the upper region 24A and the lower region 24C, and continues to remain in an insulator state. The central region 24B has a phase transition to metal. The MIT material layer 18 that includes an insulator region after the MIT is called a non-uniform MIT material layer. However, an MIT material to be used as the MIT material layer 18 usually needs to be uniform. That is, the MIT material layer 18 needs to be a uniform MIT material layer that is entirely transformed into a metal layer after the MIT.
Due to several limitations in typical fabrication methods, the MIT material layer is often non-uniform in actual industrial practice. For instance, the inventors of this patent application reported this exemplary case in New J. Phys. Vol. 6, p. 52, 2004. It was experimentally verified that the MIT material layer 18 after the MIT was easily deteriorated due to the non-uniformity characteristic. In detail, the non-uniform MIT material layer 18 was easily deteriorated due to heat generated by a large amount of current.
In order to implement an MIT in other application fields, a large amount of current needs to flow uniformly after a phase transition from an insulator to a metal occurs. Hence, it is generally essential to develop a uniform MIT material layer. A method of reducing the deterioration of an MIT material layer when current flows through an MIT device has not yet been developed.